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Arbuscular mycorrhizal fungi (AMF) provide plants with vital mineral nutrients and co‐exist inside the roots alongside a complex community of bacterial endophytes. These co‐existing AMF and bacterial root communities have been studied individually and are known to be influenced in structure by different environmental parameters. However, the extent to which they are affected by environmental parameters and by each other is completely unknown. The current study addressed this knowledge gap by characterising AMF and bacterial communities inside plant roots from a natural and an agricultural ecosystem. Using multivariate modelling, the relative contribution of environmental parameters in structuring the two communities was quantified at different spatial scales. Using this model, it was possible to then remove the contribution of environmental parameters and show that the co‐existing AMF and bacterial communities were significantly correlated with each other, explaining up to 36% of each other's variance. Notably, this was not due to the presence of know AMF endobacteria, as removal of endobacterial reads maintained the significance of correlation. These findings provide the first empirical evidence of a selective and bi‐directional relationship between AMF and bacteria co‐inhibiting plant roots and indicate that a significant fraction of this covariation is due to biological and ecological interactions between them. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Rasmussen et al., 220: 1247–1260. [ABSTRACT FROM AUTHOR]

Plant-soil microbial interactions have moved into focus as an important mechanism for understanding plant coexistence and composition of communities. Both arbuscular mycorrhizal (AM) as well as other root endophytic fungi co-occur in plant roots, and therefore have the potential to influence relative abundances of plant species in local assemblages. However, no study has experimentally examined how these key root endosymbiont groups might interact and affect plant community composition. Here, using an assemblage of five plant species in mesocosms in a fully factorial experiment, we added an assemblage of AM fungi and/or a mixture of root endophytic fungal isolates, all obtained from the same grassland field site. The results demonstrate that the AM fungi and root endophytes interact to affect plant community composition by changing relative species abundance, and consequently aboveground productivity. Our study highlights the need to explicitly consider interactions of root-inhabiting fungal groups in studies of plant assemblages. [ABSTRACT FROM AUTHOR]